The present invention relates in general to data communication circuits, and in particular to a pulse amplifier/line driver circuit for driving digital transmission lines.
Designing a data communication transmitter to drive E1 or T1 digital transmission lines that meets both the required electrical interface specifications as well as the return loss specifications has proved to be a challenging task. The electrical interface and return loss specifications are governed by C.C.I.T.T. standards G.703 and G.775, respectively. On the transmitter side, the electrical interface specifications dictate the amplitude, speed and the shape of the transmit signal, while the return loss specifications dictate the degree of impedance matching between the driver and the load.
FIG. 4 shows the template defining the transmit signal pulse shape for E1 transmission according to C.C.I.T.T. G.703 standards. Lines 400 and 402 define the template for a positive transmit pulse 401, while lines 404 and 406 define the template for a negative transmit pulse 405. The template shown in FIG. 4 sets an absolute minimum slew rate of about 30 v/.mu.sec. for E1/T1 transmit pulses. Typical E1/T1 line driver circuits, however, exhibit slew rates closer to 100 to 120 v/.mu.sec. Generally, the slew rate for a particular transmission system depends on the associated template and can be expressed in terms of peak voltage and the data rate as follows:
Slew Rate=K (Vpeak.div.1/f.sub.0)
where f.sub.0 is the data rate (e.g., 2 Mbits/sec. for E1), Vpeak is the peak voltage defined by the template, and K is a proportionality constant. The constant K is dependent on the pulse shape and template characteristics. Some typical values for K are 1 for non-return to zero (NRZ) data and 2 for return to zero (RZ) data such as E1 and T1 data.
Return loss generally refers to the amount by which the undesired return (or reflected) transmit signal is attenuated. The C.C.I.T.T. G.775 require 10 dB return loss in the frequency range from 50 KHz to 1.024 MHz, 14 dB return loss in the frequency range from 1.024 MHz to 2.048 MHz, and 12 dB return loss in the frequency range from 2.048 MHz to 3.076 MHz. Signal reflection occurs when the transmission line is not properly terminated or driven. That is, to minimize signal reflection, the transmission line must be driven and terminated with an impedance that matches the characteristic impedance of the line. Return loss specifications thus require good matching between the impedance of the driver and the load.
The above electrical interface and return loss specifications lead to several conflicting requirements and conditions. On the one hand, to maximize return loss, the output impedance of the driver inside the transmitter must be minimized. This is required to minimize the adverse affects the driver output impedance will have on the required impedance matching. On the other hand, the electrical interface specifications, or the template that defines the shape of the transmit pulses, require very fast rise and fall times. This translates to very fast positive and negative slew rate requirements for the driver circuit. In most instances, however, even a fast driver runs into slew rate limitations in the presence of fast enough input signals. When the driver experiences slew rate limiting, the driver output is not capable of following the input, and the feedback path around the driver is temporarily broken. With no feedback, the effective output impedance of the driver is no longer reduced by the gain of the driver stage. Thus, the driver exhibits large output impedance during slewing. Another condition under which the driver may exhibit large output impedance is when there are no input pulses (i.e., when the input is at ground or binary-zeros are transmitted in AMI coded signals). If the driver is designed such that its output devices shut off in the absence of an input pulse, the output will turn into a high impedance node. To maintain the output voltage at zero volts in the absence of input pulses, one approach employs an auxiliary set of output devices connected in parallel with the main output devices. The parallel output path is then allowed to draw a small amount of current to force the output voltage to zero. The auxiliary devices, however, are usually much weaker and smaller than the main output devices. As a result, the output impedance is high even if the feedback loop is intact. Large driver output impedance in turn disturbs the impedance matching of the line and reduces transmit signal return loss.
Another factor affecting the design of the driver is signal amplitude requirements. The minimum required amplitude for the transmit signal at the end of the transmission line is set at .+-.3.0 volts on twisted pair wire and .+-.2.37 volts on coaxial cables in T1/E1 data transmission. This translates to rail-to-rail voltage swing requirements at the output of the driver. A driver with large output swings typically requires large dynamic range and gain/bandwidth. This usually results in large current requirements and therefore faster amplifiers. While the signal amplitude and rise/fall time requirements point toward designing a very fast driver, noise consideration dictate otherwise. Generating a transmit signal with sharp transitions gives rise to higher harmonics in the frequency spectrum. The high frequency side lobes increase crosstalk noise and electromagnetic interference. Sharp signal edges also increase the possibility of overshoot and ringing which may cause deviations in the shape of the transmit signal from the specified template. Further, variations in temperature and integrated circuit processing cause variations in the speed of the driver stage. Changes in speed of the driver in turn create template matching problems.
There is, therefore, a need in data communication systems for a driver circuit that is capable of meeting the return loss requirements as well as the electrical interface specifications.